1. Field of the Invention
The present invention relates to molding materials based on polymers or polymer blends which contain, in admixed form and in sufficient amount, a dull molding material based on a graft polymer as dulling agent and which can be processed to give shaped articles having low surface gloss.
2. Description of the Related Art
Copolymers of alkenyl aromatics and nitriles and/or esters of acrylic acid or methacrylic acid are widely used for the production of industrial shaped articles. Since the shaped articles are often also intended to be impact-resistant, the stated copolymers are then mixed with rubber-modified copolymers in which monomers forming hard polymers, such as styrene, methylstyrene, acrylonitrile, methyl methacrylate, are grafted onto an elastomer. Polymers having a hard phase and a soft or rubber phase result. When the rubber phase is the grafting base, the elastomers used in particular are polybutadiene, ethylene/propylene copolymers, ethylene/propylene diene copolymers and elastomeric acrylate copolymers. The resulting polymer blends have long been commercially available as ABS, AES or ASA polymers.
For some uses, however, they have the disadvantage that shaped articles produced therefrom possess high surface gloss and reflect incident light.
It is known that the often undesirable surface gloss of the plastics shaped articles can be reduced by providing their surface with embossed structures or a dull coating or adding inorganic fillers, crosslinked polymers or rubber components into the plastics molding materials forming them. The methods are unsatisfactory since some of them are labour-intensive and some lead under the processing conditions to nonuniform surfaces or to shaped articles having insufficient impact strength. DE-A 42 31 995 describes dulling agents comprising methacrylates polymerized in emulsion. They contain no rubber, are brittle and adversely affect the toughness of the end products. The preparation of dull graft copolymers based on polybutadiene has been known for some years from EP-A 0201 099, EP-A 0328 960 and EP-A 0381 358. According to EP-A 0381 358, vinylaromatics are subjected to mass polymerization and then to suspension polymerization with unsaturated nitrites in the presence of from 5 to 15% by weight of a special polybutadiene to a conversion of from 1 to 15% by weight of the amount of monomer. The disadvantage of this process is that a higher concentration of polybutadiene leads to problems, and the process functions only with special polybutadienes having a low cis content but not with other polybutadienes and also not with styrene-containing butadiene rubber (SBR). Furthermore, maintaining the low conversion limits in the mass polymerization is extremely critical for achieving the desired results. The dulling agents obtained according to EP-A 0201 099, 0328 960 and 0381 358 all have the serious disadvantage that they are not weathering-resistant and become discolored and their impact strength declines.
U.S. Pat. No. 4,857,591 discloses the use of a hydrogenated butadiene/acrylonitrile copolymer as a dulling agent for polymers. However, such hydrogenated copolymers often still contain residual C.dbd.C double bonds, as a result of which the weathering stability of the copolymers is adversely affected. These dulling agents, too, have a relatively dark color which is evident in the end products.